A variety of prosthetic valves have been developed for replacing defective native valves, such as an aortic heart valve, in a human body. Prosthetic valves typically include a valve structure mounted on a stent which is delivered to a treatment site via a percutaneous catheterization technique. A stent is a generally cylindrical prosthesis introduced into a lumen of a body vessel via a catheterization technique. Stents may be self-expanding or balloon expandable. Balloon-expandable stents are typically crimped from an initial large diameter to a smaller diameter prior to advancement to a treatment site in the body. Before crimping, a balloon expandable stent is typically placed over an expandable balloon on a catheter shaft.
To properly position a balloon expandable stent on a delivery catheter over the expandable balloon, the stent must be smoothly and evenly crimped to closely conform to the overall profile of the catheter and the unexpanded balloon.
Despite the most careful and firm crimping, physical properties of the material used in manufacturing stents (some stainless steels, tantalum, platinum or platinum alloys, CoCr, MP35N or shape memory alloys such as Nitinol™) allow a certain amount of “recoil” of the stent. That is, the stent tends to slightly open up from its crimped diameter once the crimping force has been removed. In some instances, the stent diameter has been shown to increase about 15% from its crimped diameter.
The enlarged recoil diameter increases the overall profile of the underlying catheter and balloon. Thus, since the stented prosthetic valve is configured to be delivered percutaneously, in a less invasive procedure, a smaller device is beneficial.